In high speed data transmission systems such as a wireless LAN (WLAN) and various digital subscriber line (xDSL) networks, when a parallel communication mechanism with multiple sub-channels is adopted, it is needed to perform bit and power allocation on the sub-channels appropriately in order to transmit bit streams efficiently with less errors.
At present, bit and power allocation methods based on water-filling principle (see Tong Xuejian and Luo Tao, “OFDM mobile communication technology principle and application”, People's Posts and Telecom Press) and bit and power allocation methods derived from water-filling principle with improvements, such as the Chow algorithm (see P. S. Chow, J. M. Cioffi and J. A. C. Bingham et al., “A practical discrete multitone transceiver loading algorithm for data transmission over spectrally shaped channels”, IEEE Trans. Commun., Vol. 43, No. 2, February, 1995), an LC algorithm based on a greedy algorithm (see J. M. Cioffi, “Advanced digital communication”, course reader [online]. URL: http://www.standford.edu/class/ee379c (accessed finally on May 20, 2010)) and etc., are widely used in multicarrier systems like orthogonal frequency division multiplexing (OFDM), Multiple-Input Multiple-Output-OFDM (MIMO-OFDM), Discrete Multi-Tone (DMT) etc. All these methods use the gap approximation of a QAM modulation format (see John M. Cioffi, “A multicarrier prime” [online]. URL: http://www.stanford.edu/group/cioffi/documents/multicarrier.pdf (accessed finally on May 20, 2010)) and a given symbol error rate (SER) to provide a relation between the power and the number of bits.
There exist two drawbacks in doing so: on one hand, the gap approximation with respect to a square QAM modulation format per se is not accurate enough; and on the other hand, the symbol error rate can not reflect the system performance precisely, while a bit error rate (BER) is the final reflection of the system performance.